This issue could be caused by any older plugins that require a specific version of SWIFT_VERSION. It would be be helpful to evaluate your plugins for any such.
If you are not sure, it might be worth to take a look at your project. I would encourage you to contact the Appflow support for further assistance.
Then in my case I had another plugin “cordova-plugin-photo-library” which required an old version of “cordova-plugin-add-swift-support”.
So I uninstall both plugin an reinstall first “cordova-plugin-add-swift-support” then “cordova-plugin-photo-library”.
And don’t forget to delete all plugins and platforms from your project. Then try to build.
It works for me.